1. Field of the Invention
The present invention relates to a cleaning apparatus and a cleaning method, and more particularly, to a cleaning apparatus and a cleaning device for, for example, removing foreign matter by uniformly irradiating the entire substrate to be cleaned in a cleaning tank with ultrasonic waves.
2. Description of the Related Art
FIG. 11 is a schematic cross-sectional view of a conventional cleaning apparatus for cleaning a semiconductor wafer in a chemical tank by utilizing ultrasonic vibrations. In this cleaning apparatus, an inner tank 2 (generally made of quartz) is charged with a chemical 19 prepared for removing foreign matter or contaminants from a substrate to be cleaned 4, for example, a mixture of NH.sub.4 OH, H.sub.2 O.sub.2 and pure water. The cleaning chemical 19 overflowing from the inner tank 2 is received by an overflow tank 3 provided outside of the inner tank 2. The chemical 19 in the overflow tank 3 is filtered by a filter (not shown), and then returned to the inner tank 2 by a circulating pump (not shown). The chemical 19 in the inner tank 2 is thus kept clean. If it is desired to keep the temperature of the chemical 19 in the inner tank 2 from 40.degree. C. to 70.degree. C., a heater (not shown) sealed by a quartz tube may be disposed within the inner tank 2.
Both the inner and overflow tanks 2 and 3 are held in an outer tank 9. A liquid serving as the ultrasonic wave transmission medium 8, for example, water, is present at the bottom of the outer tank 9. The bottom surface of the inner tank 2 is in contact with this ultrasonic wave transmission medium 8. At least part of the bottom surface of the outer tank 9 is an ultrasonic wave radiating plate 6. On the undersurface of the radiating plate 6 is provided a plurality of ultrasonic wave oscillators 7. A wafer basket 1 holds a plurality of substrates 4 to be cleaned in a vertical direction as viewed in FIG. 11. The wafer basket 11 is positioned by a basket guide 5 in the inner tank 2.
In the conventional cleaning apparatus arranged in the manner described above, to clean the substrate 4 ultrasonic waves are irradiated into the ultrasonic wave transmission medium 8 from the ultrasonic oscillators 7 through the ultrasonic wave radiating plate 6. The irradiated ultrasonic waves pass through the bottom of the inner tank 2 and are transmitted to the chemical 19. At that time, ultrasonic energy vibrates the chemical 19 at a frequency determined by the ultrasonic oscillators 7. The vibrated chemical 19 physically acts on the surface of the substrate 4 to be cleaned. Hence, the surface of the substrate 4 to be cleaned is sufficiently cleaned due to a combination of chemical cleaning action of the chemical 19 and physical cleaning action of the ultrasonic vibrations.
The lower portion of the wafer basket 1 is generally narrowed to form a wafer supporting portion 20 for holding the substrate 4 to be cleaned. The wafer supporting portion 20 damps ultrasonic vibrations which are transmitted from the bottom surface of the inner tank 2 in an upward direction into the chemical 19. More specifically, ultrasonic vibrations are not transmitted sufficiently to peripheral areas 10 of the substrate 4 to be cleaned which are located above the wafer supporting portion 20, as shown by the X marks on the substrate 4 to be cleaned of FIG. 11. Therefore, the peripheral areas 10 of the substrate 4 to be cleaned are not subjected to the physical action of the ultrasonic vibrations and thus are not sufficiently cleaned, as shown in FIG. 12. FIG. 12 shows how the substrate 4 to be cleaned is cleaned with time. Cleaning of the peripheral areas 10 due to the chemical action of the chemical 19 alone takes a long time.
The above-described type of conventional cleaning apparatus and cleaning method have a drawback in that ultrasonic vibrations are not sufficiently transmitted to the peripheral portion of the substrate 4 to be cleaned and the entire substrate 4 to be cleaned cannot thus be cleaned uniformly, efficiently and thoroughly.